Hygrometer of high sensitivity



HYGROMETER OF HIGH SENSITIVITY Filed Dec. 17, 1956 William D. Weston, Blisworth, and John microscopic examination.

Wed SP iem HYGROMETER OF HIGH SENSITIVITY Herbert, Horton, England, Limited, llford, England, a British company Application December 17, 1956, Serial No. 628,936

Claims priority,,application Great Britain December 12, 1955 2 Claims. Cl. 33835) assignors to The Plessey Company the To this mixture was added a I dium' sulphate having a volume approximately 6 times that of the mixture. The sodium sulphate was comon a 44 mesh screen and none passed through a 60 mesh screen. i

The mixture was packed into perforated metal moulds and heated in a near saturated solution of sodium sul- ..phate to decompose and xanthate and leach out most'of the sodium sulphate. The residual sodium sulphate was quantity of finely divided carbon (or other conductive material) is dispersed in an insulating material which can absorb moisture from the air, with a consequent change in dimensions, the resulting body will be electrically conductive and itsohmicresistance will be a function of the humidity of the atmosphere to which it is exposed.

It is a featureof this invention to make such a conductive body in a highly porous form with a maximum area per unit volume, so that the exchange of moisture between the body and the air may take place very rapidly. By this means an improved humidity-responsive element, the resistance value of which can be used 7 as a measure of atmospheric humidity, and which re applying one of the known techniques for forming a solid foam from the material. Thin sections are then cut from the foam using a microtome andemploying the technique usual for cutting biological sections for The sections, which preferably are .5 to 50 4 thick, are thenmounted between suitable metal electrodes to form resistive elements.

Example I.--Alkaline xanthated cellulose derived from 100 parts by weight of dry raw cellulose was mixed with 10 parts of finely chopped hemp fibre, 55 parts gas black, 500 parts hydrated sodium sulphate (ground to pass through 50 mesh per inch screen) and suflicient saturated sodium sulphate solution to render the mixture mouldable. The mixture was packed into perforated moulds and heated in a sodium sulphate solution to decompose the xanthate and leach out most of the sodium sulphate. The crude sponge thus obtained was then treated with acid, the residual sodium sulphate was washed out with water and the remaining sponge dried. The dried sponge was immersed in molten parafiin 1 wax at 80 C. and cooled to room temperature. The waxed block was suitably shaped, clamped on a micro-' tome and 0. 002 slices were cut from it. The slices were washed in xylene toremove the wax-and then successively with ether, alcohol and water. 'The cleaned slices were held between two brass clamps about 1" wide with a gap of ,4," between them. The resistance between the clamps was about 0.5 megohm at zero humidity increasing to 1.5 megohms at 100% relative humidity. The change in resistance was found to be substantially complete within 15 seconds of changing the humidity.

Example II.Xanthated cellulose derived froin' 100 parts by weight of dry raw cellulose was mixed'with' 25 parts by weight of undissolved cellulose fibres and- 60 removed by washing in several changes of vwater and the remaining sponge dried.

The dried sponge was immersed in molten parafiin wax at 60 C., de-aired and cooled to room temperature. The waxed block was suitably shaped, clamped 'on"a microtome and 0.003" slices were cut from it. The slices were washed in xylene to remove the wax and then successively with ether, alcohol and water. The cleaned slices were held between two brass clamps about 1 wide with a gap of between them. With a typical slice at +20 C. the resistance between the clamps at different values of relative humidity is given in the table.

Changes in resistance were found to be substantially complete within 15 seconds of changing the humidity.

One form of moisture measuring apparatus intended to measure the humidity of air is schematically illustrated in the accompanying drawing. The moisture sensitive porous element according to the invention is indicated at I. It is in the form of a thin slice shown considerably enlarged, and is held along two opposite edges by brass clamps 2 and 3. Only part of the clamps has been shown assuming that, as the above indicated, the width of the clamps is about 8 times their distance. In order to utilise the variation of resistance of the element 1 ac cording to the humidity of the air for an indication, the elementl is arranged in a bridge circuit, in which it is connected across a constant voltage from a source B, in series with a fixed resistor 4; a second branch of the bridge comprising fixed resistors 5 aand 6 is connected across the same voltage in parallel to the elements 1 and 4. A voltmeter 7 of adequate sensitivity and high internal resistance is connected between the junction point 8 of elements 1 and 4 and the junction point 9 of elements 5 and 6. It should be understood that this arrangement has only beenv described by way of an example and that many other circuit arrangements by which the variation in the resistance of element 1 can be utilised for obtaining a visual indication, or a con-..

Patented Mar. 22, 1 960.

quantity of hydrated so thated 'cellulose and a'dispersion in the foamed struc hire of elemental carbon particles.

2. A humidity sensitive element comprising a selfsupporting. unit consisting of elemental carbon particles dispersed in. an electrically insulating solid foamed Xan-... th'ated cellulose, the thickness of the self-supporting unit' being within the range of 2.5 and 50 microns.

References Cited in thefile of thispatent- 4 Tate Aug. 12, 1941 Ga'ut'et al. Sept. 23, 1941 Lichtgarn Sept. 13, 1944 Bouyoucos Jan. 16, 1945 Vander Kroon Dec. 6, 194-9 Gurewitsch Oct. 7, 1952 Stearns et al. Nov. 1,' 1955 White Sept. 17, 1957 Seehof et a1. Sept. 15, 1959 OTHER REFERENCES Microscopical Section Cutting (March), second edition, published by J and A Churchill (London), 1882 (pages 34, 35 and 36 relied upon). 

1. A HUMIDITY SENSITIVE ELEMEENT COMPRISING A SELFSUPPORTING UNIT HAVING A THICKNESS WITHIN THE RANGE 0.5 MICRON TO 50 MICRONS, THE UNIT CONSISTING OF A MOISTURE ABSORBENT ELECTRICALLY INSULATING SOLID FOAMED XANTHATED CELLULOSE AND A DISPERSION IN THE FOAMED STRUCTURE OF ELEMENTAL CARBON PARTICLES. 